The present invention relates to optical fiber technology and, more particularly, to optical isolators useful in blocking reflections of optical signals generated by a laser through an optical fiber.
A recurring problem in optical fiber networks is the dispersion of the optical, i.e., light, signal in the optical fiber. Through dispersion the different components of an optical signal are spread out. For example, distance can cause dispersion of a signal traveling along an optical fiber. If left unchecked or uncompensated for, what was once a sharp signal at transmission arrives as a blurry or indefinite signal at reception. A digital signal train of logic "ones" and "zeroes" may arrive as an indistinct series of signal oscillations between "three-fourths" and "one-fourth" logic values.
Dispersion can arise from many different sources. Many of these problems have been essentially solved or avoided. However, one source which has been largely overlooked thus far is one of the common elements in a fiber optic network, the optical isolator.
In present day optical fiber technology, semiconductor lasers are typically used to generate and to relay light signals on optical fibers. These lasers are particularly susceptible to light signal reflections, which cause a laser to become unstable and noisy. Optical isolators are used to block these reflected signals from reaching the laser. Ideally, these optical isolators transmit all of the light signals in the forward direction and block all of the signals in the reverse direction.
Many of these optical isolators use the polarization modes of a light signal as a mechanism to block reflected signals. The present invention recognizes that as distances over which optical signals are transmitted lengthen and as signal transmission rates increase, these optical isolators may create dispersion problems. Stated differently, with the wide application of optical isolators in present day, high-speed optical communication networks over long distances, the problem of polarization mode dispersion is expected to become more evident.
Optical devices which addressed this problem were described in a patent application entitled, "AN OPTICAL DEVICE WITH LOW POLARIZATION MODE DISPERSION," Ser. No. 08/006,893 (now abandoned), filed Jan. 21, 1993 by two of the inventors of the present patent application and assigned to the present assignee. The present invention also solves or substantially mitigates this problem by providing for a dual stage optical isolator with reduced polarization mode dispersion.